Just tried Guest Additions from SlackBuilds.org for Slackware 13.37 after uninstalling original. virtualbox-kernel-addons seems to have succeeded, but virtualbox-addons failed on installpkg (grep and chroot).

I noticed that the original installer was not able to build kernel modules due to missing headers. Maybe that's the issue, how would I install kernel headers?

I dont think you can mount that on boot because the guest module loads after all the startup process has almost finished. I think it loads right before X launches. In my experience, When I have used this, it has been mounted manually. Another option is to modify your line in fstab and add 'noauto' so that it does not try to mount it on boot, but you can still just mount it as you need it without having to type the whole command.

Looks like you're right, none of the solutions in that link worked for me. Guess I'll just copy and paste for now.

I'm curious to see if successfully building the Guest Additions kernel modules will make a difference. Problem is, I don't know how to install the kernel headers VirtualBox needs when kernel-headers is blue and locked. Removing it from excludes and updating didn't work.

Yes, what you need is to install the 'kernel-src' package that matches your kernel version. After that, you would install the guest additions from your .run file (from vbox). Then you will need to do /etc/rc.d/rc.vboxdrv setup to compile the modules. That's how i've always done it.